Monday, October 13, 2025

Why did the Road Cross the San Andreas Fault? 23 Years of Geologic Change (a new Update)

2002
I've been leading geology field studies trips to lots of places in the American West for 37 years and started to take digital pictures in 2001. I sometimes struggle to find new things to photograph when I visit a place for the 37th time, but in some cases it is not a problem. There are geologic changes that happen on a yearly basis, and with twenty-three years of photos (minus two due to Covid), the changes become obvious. This is a continuing update from a post in 2013, and I'll probably continue updating for the foreseeable future.
2004
Highway 25 in the California Coast Ranges connects the town of Hollister with the access road to Pinnacles National Park (formerly Pinnacles National Monument). Along the way the highway crosses the San Andreas fault in a section where the fault creeps an inch or so each year (36°35'54.27"N, 121°11'40.19"W). Most years we've stopped to have a look at the effect the movement has on the pavement. In 2002 and 2004, the damage was obvious.
2008
By 2008 someone had patched the road, and no fault motion was evident.
2009
Little damage was evident in 2009 either. But by 2010 cracks had begun to appear as the fault stressed the pavement.
2010
The fact that the fault creeps in this region is a good thing. It means that stress is not building along the fault surface, but instead is being released gradually. The sections of the fault to the north and south of the creeping section are locked by friction, and are building up the ominous stress that will eventually produce quakes with magnitudes in the range of 7.5 to 8.0. The quakes are coming and we need to be as prepared as possible.
2012
By 2012, the road had been completely repaved, and  yet the shearing was already evident.
2013
It became even more pronounced by 2013 and in 2014. Just by chance, the person working as a scale was the same individual as in 2004.
2014

In 2015 the fractures were moderately larger. They'll need to start thinking of road repairs before long.
2015
In 2016 Laura once again provided scale, as she did in 2014 and 2004.
2016
Here in 2017, long-time trip volunteer Mary provides scale. The cracks in the road are just a bit larger, but they didn't look appreciably different than the previous year except for a twist (pun intended).

2017
On Dec. 2, 2018, the break to my eye seems more continuous. It's now been six years since the road was completely repaved.
2018

Last year the paint was deformed (twisted), but not split (below).
2017
The offset paint strip reminds me of illustrations of elastic rebound theory, the idea that stress builds up on a fault line over time. In that model, the land on either side of the fault is distorted over time until the frictional resistance is overcome and the rock snaps back to its original shape. That won't be happening with the paint. Last year in 2017 I said "if they don't repair the road (as they often do; see above), it will probably show a clear break by next year." Here's what transpired:

First, a close-up on 2017's center stripe...
2017
And here's how it looked on Sunday, Dec. 2, 2018:
2018
As predicted, the break in the paint is complete...

In 2019 (those last few halcyon days before Covid) long-time volunteer Paul provided scale (he has been assisting MJC with field trips for 25 years!). The crack continues to grow, and I wouldn't have been surprised if it was patched by next year.
 The paint on the center strip is split even more.
November 2019
And then Covid happened and for a few years we were not able to conduct our field studies classes. In 2022 we made a return visit with our students and here is the then-current condition of the highway. It didn't appear that any repairs have been conducted yet. Our host is once again Laura, who was with us back in 2004 and subsequent years!
November 2022
Fault creep is not a constant. I didn't see a whole lot of change over the last three years, although I didn't get as many close-up shots. Here's a closer look with Paul, our other long-time volunteer. What do you see that is different?
November 2022
In 2023 the road continued to become more deformed, and the passing traffic produced an audible thump as it passed over the fault. Our host since 2004, Laura, was not able to join us, so her husband Ryan stood in her place.
Oct. 28, 2023
And then it was 2024, October 12 to be specific. Once again our guide is Laura, who has assisted on these trips since at least 2004. The crack continues with minor changes, and it may be overdue for another repaving job.
October 12, 2024
They may not have repaved the highway, but they did in fact repaint the center line, which obscures about two inches of right lateral offset.
And that brings us to 2025 (October 11). Our long-time volunteers weren't on the trip this year, so River and Makayla jumped in to provide scale. 

Oct. 11, 2025
The road hasn't been repaved since 2012, and the crack is continuing to slip and fracture. The pavement at the road median has fragmented, and completely repainted which hides the offset a bit.
These little changes that happen at a rate visible in human lifetimes add up to huge changes when multiplied by thousands or millions of years. The nearby eroded volcano of Pinnacles National Park has been displaced 195 miles (315 kilometers) in the last 20 million years or so by movement along the San Andreas.

How will it look next year?



Thursday, October 9, 2025

The Things We Don't Think About: The Aftermath of a Major Earthquake from a Great Valley Perspective

The Great Shakeout is planned for Oct. 16, 2025, at 10:16 AM. 

Source: U.S. Geological Survey

We Californians live in earthquake country. The state is riddled with fault zones, and many of them are more than capable of producing damaging quakes. Some of the state's biggest cities are built on or adjacent to active faults, and much attention has been devoted to the probabilities and outcomes of major quakes in those cities, with predictions of possibly thousands of fatalities and hundreds of thousands of injuries. The dystopian models predict legions of newly homeless people, and badly disrupted services and ruptured transportation lines. It's grim reading. Government organizations are cognizant of the danger, and billions of dollars have been spent on preparing for such quakes, including reinforcement of vulnerable infrastructure, and educating the population about the dangers.

But what about the outlying regions of the state, somewhat removed from the most damaging aspects of the coming quakes? What is life going to be like after the fabled "BIG ONE" strikes? That's what I want to write about, since that's where I and many of my friends and acquaintances live. What's going to happen to us here in the Great Valley, in towns like Modesto and Turlock?

Source: Earthquake Country Alliance
First, the good news: the threat of severe shaking is considerably less in much of the Great Valley, with certain important exceptions (see the map above). Much of the valley floor is located tens of miles east of the most dangerous faults like the Hayward or San Andreas. The expected shaking will lead to some damage, but for the most part, buildings and infrastructure (roads, rails, pipelines) should fare reasonably well.

Liquefaction damage in San Francisco, 1906. Source: USGS
The sediments of the Great Valley can have a tendency towards liquefaction, the phenomenon of turning into "quicksand" during shaking, causing buildings to partially sink and severing buried pipelines and cables. This is especially true along river floodplains, including the San Joaquin River and within the Sacramento Delta (citizens of Tracy and Patterson need to take special note). This is the result of a shallow groundwater table. Luckily, much of middle and eastern parts of the valley have a deep water table, and liquefaction is less likely in the dry sediments.

Here's what happened last time in 1906. Source: Wikipedia open domain
So...let's imagine the worst has happened. A 7.8 magnitude earthquake has struck in the Bay Area, and devastation is widespread throughout the region. Hundreds, perhaps even several thousand people have died, and many tens of thousands have been injured. There are widespread power outages, and pipelines have been broken in many places, leaving many without water. Streets and freeways are blocked by debris and collapsed bridges, and emergency services are overwhelmed. Several hundred thousand homes have been rendered uninhabitable. Fires are burning in many areas. Dozens of aftershocks, some as great as magnitude 6, cause additional damage and panic. Life has taken a catastrophic turn, and millions are affected in profound ways. There's much uncertainty now as the population faces concerns of water, food, shelter, and danger from further seismic activity. It's the time that we find out if years of planning and preparation have been effective. Emergency services swing into action.

Agricultural fields in the western part of the Great Valley near Patterson (my photo)
Meanwhile in Modesto the situation is quite different. Although heavy shaking is felt by literally everyone, the sensation is more of a rolling motion, not the fierce movements that brought down bridges and unreinforced buildings in the Bay Area. Some chimneys are shaken loose, cracks occur in some older buildings, and items fall from shelves throughout the city. Some people are hurt by falling debris, but deaths are probably few in number. Electricity goes out due to the disruption statewide to the state power grid. The outages mean that water pressure can't be maintained, so faucets are dry throughout the city. Cell phone towers may have been damaged, and those that aren't are soon overwhelmed by phone traffic (it has been said that social media spreads earthquake news faster than the seismic waves themselves).

In the immediate aftermath, life has become inconvenient. The power is out, the water is off, sirens are blaring all over the city, but for the most part it's a time of waiting. Waiting for the power to come back on, the water to flow, and the phones to start ringing again. It could take days, maybe even a week or two. All things considered, for most of us it is an interruption.

But there are larger consequences with this incredible earthquake in places like Modesto. 

Emergency services will be largely unavailable in town. Ambulance and EMT crews will pressed into service in the Bay Area, and local hospitals will be filled to overflowing with the injured (many Bay Area hospitals will be overwhelmed and maybe even unusable). The same will be true of firefighters and perhaps law enforcement personnel as well.

Many Bay Area freeways will be impassable, but emergency supplies must still reach the region. Interstate 5 may still be open, but Highway 99 will become a more critical lifeline than it is today. Traffic will increase. With refinery production in the Bay Area interrupted, gasoline will become scarce. Expect huge price increases and severe shortages at filling stations across the state.

As much as I don't like to think about it, most people will not have prepared for the eventuality of earthquakes. They won't have supplies of water and food and other materials on hand, so local store shelves will quickly empty out, and with a severely disrupted supply chain, those shelves will not quickly refill. There will be shortages. Restaurants and fast-food outlets will likely be closed for extended periods.

It's a given that our local community, our officials and authorities and our relatively undamaged infrastructure will be an important part of recovery efforts in Central California. My own institution, Modesto Junior College, has plans in place for sheltering and supporting thousands of people made homeless by the quake. They will likely be our guests for weeks or months. Many other schools and institutions have similar plans. The influx of hurting and anxious people will no doubt place a huge strain on our infrastructure, and I fervently hope we can rise to the occasion, just as I would hope that others will look upon us with compassion in case of catastrophic flooding in our valley (that's an entirely different subject for a future blog).

It's going to be a challenging time, and we can respond to it in two ways:

We can ignore the entire possibility of large earthquakes, and go on living our lives as if nothing will ever happen. That is frankly what most of us are doing now. It's seemingly worked well so far. We are of course living among fault zones that have been building up stress for a century or two, and they are all capable of producing damaging earthquakes right now. I don't think of this as a good choice. Not at all.

An aerial view of a devastated San Francisco shortly after the earthquake in 1906 (USGS)
Or...we could prepare. Take the possibility of a major quake with the intensity you would give it if you knew it was happening a few days. Gather supplies of food, water, and first aid supplies right now, before the shelves emptied by desperate quake refugees and victims. To the extent possible, keep your gas tank closer to full than empty. Have emergency go-bags ready in case you have to evacuate your home or community. Develop an emergency plan for your family, and maybe your neighborhood. Learn about the known seismic hazards in the place where you live. Check out my previous blog for more links and ideas. There is a great resource centered on Central California published by the California Earthquake Authority, the Earthquake Country Alliance, and the California Governor's Office of Emergency Services, that can be reached by clicking here.

It would be nice if we could know in advance that a quake is about to hit, but we don't. Since that's the case, we should be as ready as possible.

Source: USGS




Tuesday, September 16, 2025

What If an Earthquake was Predicted to Happen in Four Days? What Would You Do??

 The Great Shakeout is planned for Oct. 16, 2025, at 10:16 AM. 

And why aren't you doing it now?!

Art by Zeo

NOTE: THIS IS HYPOTHETICAL, NOT A REAL PREDICTION!

It is reported that there is a 50% chance that your region is going to be hit with a magnitude 7.5 earthquake within the next four days. What are the most important things you would do in response to this prediction? What questions would you ask?

This is an exercise I do with my students and their responses are quite extensive, and exceedingly logical. They speak of storing up water and food supplies, checking their first aid kits, preparing family emergency plans, and oftentimes, evacuating the region and getting away from the quake damage.

They are great ideas, but some thoughts arise as they consider their options...

  • If people know an earthquake is coming, there would be chaos in stores as people hoard up emergency supplies. Remember the shortages during the Covid epidemic. Or any hurricane in the last few years. 
  • Traffic will be horrific, and where exactly would hundreds of thousands, or even millions of people go?
  • And...how many people would actually question the source of the prediction and its reliability. Predications have caused panic in the past. We've never been able predict earthquakes on short notice before, so why would this prediction be different?

The San Andreas fault in the Carrizo Plains of Southern California

It's a real problem because millions of people in California and other parts of the country live with the threat of earthquakes. And few of them have given thought to what they would do, and they are not in any prepared for an actual catastrophe. How many people have stored up emergency water and food supplies, and how many actually have an emergency plan?

So my response to my students is this:

We cannot know precisely when an earthquake will strike, nor where. We know the potential for earthquakes for many areas, especially in California, and in many cases have a broad idea about the probability of a quake taking place within a few years or decades. 

If you live in area with possible earthquake activity, you must prepare beforehand. Otherwise you'll be a victim. The California Earthquake Authority has lots of information about how to prepare, but let my students give you the short list:

  • Become familiar with the risk of quakes where you live. Know what possible level of magnitude and shaking you could face.
  • Have emergency supplies of water and nonperishable food sufficient for at least a week
  • Have a first aid kit handy and know how to use it (emergency services will be overwhelmed)
  • Consider how you will deal with the loss of power, phone service, and water for at least week
  • Have a go-bag of clothing, medicines, important documents in your vehicle or at least close at hand (a good idea for many kinds of emergencies)
  • Have an emergency plan in place for all members of your household

Be prepared!!


Sunday, August 3, 2025

Journeys in the Back of Beyond: Wait, This is a National Park?

 

Pack up your stuff kids! We're going to a NATIONAL PARK!

Cool! Does it have big mountains?

No.

Oh, does it have deep canyons?

No.

Can we go swimming in a lake or river?

No.

Oh, will rangers give an interesting talk around a campfire?

No, there's no campgrounds.

Does it have trees at least?

Well, yeah, but they're all dead.

What kind of a national park is this???

It is of course the national park dedicated to a bunch of long dead trees, Petrified Forest in Arizona. It was the third day of our recent exploration of the Back of Beyond, the vast geological wonderland of the Colorado Plateau. We had traveled east of Flagstaff to explore the prairies and badlands that make up much of that strange landscape. 


To most folks it's all about the petrified wood. Some might even think it's a one-note kind of place, of some interest maybe, but not worthy of a full exploration. They couldn't be more wrong! The park is a showcase of geology, archaeology, and paleontology. And it has some unique and dramatic scenery to boot.

The wood was discovered first by indigenous people thousands of years ago (evidence indicates a human presence in the region for at least 12,000 years). They made use of the petrified wood as projectile points, knives, scrapers, and even building materials (there is a reconstructed pueblo in the park made almost entirely of the agatized wood).

Petroglyph at Newspaper Rock in Petrified Forest National Park

The European colonists who always get credited for their "discoveries" came along in the 1850s. The military investigated the region first, followed by ranchers and homesteaders. The area became better known as railroads were built through the region, taking advantage of the relatively flat terrain and availability of water along the Puerco River. A hotel stop at Adamana allowed passengers to tour what was then known as Crystal Forest, and walk off with tons of petrified wood samples. Later on a stamp mill was constructed to grind up the wood to be sold as an abrasive. It concerned some people.


Often the creation of a national monument, which can be established by presidential proclamation, is a controversial affair. Such actions were taken by presidents to protect endangered resources and archaeological sites that locals were often profiting from. It's only as tourism follows that proclamation that local folks begin to accept the presence of a monument in their midst. The story seems to be somewhat the opposite in this case, as the Arizona legislature unsuccessfully proposed a Petrified Forest National Park as early as 1895 in response to local concern about the disappearance of vast amounts of petrified wood. The passage of the Antiquities Act in 1906 gave Theodore Roosevelt the power to designate Petrified Forest as the nation's second national monument that same year. Congress waited until 1962 before "upgrading" the monument to national park status (monuments have the same level of protection that national parks do, although funding sometimes lags behind). Subsequent actions in 2004 more than doubled the size of the park to 341 square miles (884 sq km). 

Despite the protective actions, it's estimated that 12 tons of petrified wood disappear from the park every year (the thieves need to read up on the curse of stolen wood). It's ironic that so much is stolen, since it can be purchased legally just outside the park.

Being petrified is not the normal fate of most trees. Most of them decay and are destroyed, but in just the right circumstances petrifaction can happen. The tree has to be isolated from the atmosphere and destructive microorganisms by being buried quickly in fine mud and clay. There needs to be a source of silica (silicon dioxide), and the movement of groundwater. At Petrified Forest, the trees were ripped from distant forests by muddy flashfloods and volcanic mudflows and transported many miles, coming to rest in river floodplains and swamps. Layers of volcanic ash provide the silica and the swamps and rivers provide the groundwater. Over time the silica fills in the cells of the wood, preserving it. Oxides of iron and other metals provide the bright coloration.


But that's not what I'm really here to talk about!

The real value of Petrified Forest National Park lies in the story contained in its multicolored strata, the Chinle formation. It records a tumultuous period in Earth's history, the middle to late Triassic Period from about 225 to 208 million years ago.

When the Chinle formation is exposed to the elements, the mud and clay is easily and quickly eroded, preventing the development of soil and inhibiting plant growth. The resulting barren desert badlands hardly look like the remains of a tropical environment of large rivers and lakes, but that is what it is. And those barren sediments hide a wealth of paleontological remains (that's short for "fossils").

Exposures of the Chinle formation in the northern portion of the park

The Mesozoic Era (252-66 million years) is often called the Age of the Dinosaurs, and of its three periods, the Jurassic is best-known (perhaps because of a certain film series), and the Cretaceous only slightly less-known, since no one ever made a movie called "Cretaceous Park". It did however gain notoriety as the ending curtain-call on the reign of the diverse and occasionally gigantic reptiles, due to a massive asteroid strike in what is today the Gulf of Mexico.

But the Triassic Period gets less attention. There are few if any dinosaurs from the Triassic in children's plastic toy collections, and yet the Triassic is the period in which the dinosaurs appeared on the planet for the first time. Few of them were large, and they didn't dominate their habitat the way their progeny in the Jurassic and Cretaceous did. There were other creatures inhabiting their world, all of whom were survivors of the greatest extinction event ever to strike our planet. And from within their ranks the first mammals emerged.

The Permian-Triassic mass extinction event was the worst devastation ever visited upon life on planet Earth, wiping out as much as 80-90% of existing species. The cause is not precisely known, but a vast outpouring of lava and ash in Siberia is thought to be a main contributor to a sudden change in the climate that caused a spike in temperatures and acidity in the oceans (carbon dioxide rose from 400 to 2,500 ppm, compared to our current historical rise from 280 to 420 ppm). A total break-down in ecosystems followed, both on land and at sea. It took millions of years for the survivors to rebuild any kind of stability (only to be disrupted again at the end of the Triassic).

And this is why Petrified Forest is so important. It gives us a picture of conditions in the middle and late part of the Triassic when some semblance of stability had returned to the world. The Chinle formation, which is exposed over most of the park, has yielded up to 200 species of plants and around 60 species of animals. Nearly a hundred of them were first discovered at Petrified Forest National Park.

The Triassic world looked little like the subsequent Mesozoic era. The Jurassic and Cretaceous periods saw the dominance of terrestrial habitats by hundreds of species of dinosaurs big and small, plant-eaters and meat-eaters alike. Mammals and smaller reptiles scurried in the underbrush while small amphibians continued to thrive in rivers and streams. 

Diorama of animals and plants of the Chinle formation, Petrified Forest National Park

The Triassic was characterized by several animal groups that competed with each other in the aftermath of the great Permian extinction. There were still large predatory amphibians up to 10 feet long, including Metoposaurus. Many years ago, one of my students found an odd specimen in the Chinle (outside the park, for the record). It looked so odd that my guesses ran the gamut from coral to alien, but it wasn't until I saw this exhibit at the park museum that I thought of Metoposaur (see a picture here and here). To be fair, some paleontologists thought of phytosaur scutes as another possibility (see below).


There were some "not-quite-reptiles" called therapsids that were once called "mammal-like reptiles" but they actually represent a distinct group of organisms that shared a common ancestry with the reptiles. They were destined to become the ancestors to the first mammals later in the Triassic. In Permian time, the Therapsids and their related groups dominated terrestrial environments with hundreds of species, but they were decimated during the extinction. A few survived into the Triassic, including the Placerias hesternus, seen below. It was a plant-eater which at up to 11 feet and a ton in weight, was one of the larger creatures of the time.

Placerias hesternus, a dicynodont therapsid, a large plant-eater

The pterodactyls made their first appearance (and no, they aren't dinosaurs either; they had a common ancestor with them). There were in fact some dinosaurs too, but they were mostly small, and most certainly didn't dominate their ecosystem. Some found in the park, like Coelophysis and Chindesaurus, were predators but being 3 to 7 feet long they were hardly the gigantic terrifying creatures of the Jurassic and Cretaceous periods.

Artist: Dr. Jeff Martz, NPS

The creatures that brought terror to the ecosystem were the Archosaurs, the reptilian line that led to the crocodiles, birds and dinosaurs. The sediments of Petrified Forest National Park have yielded up specimens of Postosuchus, which at upwards of 23 feet in length, was gigantic by Triassic standards. It was a formidable predator.

Artist: Dr. Jeff Martz/NPS

Another member of the archosaurs found in the park are the Phytosaurs. They strongly resemble the crocodiles, but are not that closely related. They are instead a marvelous example of convergent evolution, by which different lineages evolve to similar shapes (think sharks and dolphins).
Artist: Dr. Jeff Martz, NPS

Petrified Forest is far more than a collection of petrified wood pieces lying around. It's a very different world and a fascinating place to visit and explore. It's one of the best places in the world to understand the time that gave rise to the mammals, and as such, to us.

Monday, June 23, 2025

Journeys in the Back of Beyond: Simply Driving to the Bottom of the Grand Canyon (!?)


Running a geology field studies course is nothing if not stressful. Right on the heels of the most intense storm I've experienced on one of these trips, it was morning and we were on the road to the next thing. A simple thing really, just driving down to the bottom of the Grand Canyon. But I had no way of knowing it would happen, given the storms of the previous night. Had the storm affected the road into the canyon?
 
The Grand Canyon is one of the great spectacles of the Earth. Millions of people every year stand on the edge of the abyss and peer in, taking in the vivid colors and hidden depths. Tens of thousands venture down some of the few trails that reach into the incredible gorge, and several thousand raft the river. But how many drive the canyon? Did you even know you could?
And no, I'm not talking about visiting the canyon "Thelma and Louise" style...

For more than two hundred river miles, no road crosses the Grand Canyon (plus another hundred miles along the shores of Lake Mead). Navajo Bridge near Lee's Ferry is at one end, and Pat Tilman Bridge near Hoover Dam is at the other. In between in the depths of Grand Canyon there is a single road that reaches the river. It's called Diamond Creek Road, and it starts on Hualapai Nation land at Peach Springs, Arizona. It's a marvelous adventure.
The "diamond" that gives the road its name: Diamond Peak

Without even considering the philosophical objections to building roads through the wilderness world of the Grand Canyon (objections I completely agree with), there are staggering engineering barriers. There are a series of formations, including the Redwall Limestone and the Coconino Sandstone that form sheer cliffs. The locations of trails in the Grand Canyon are controlled almost entirely by the few locations where they can surmount the cliffs of these two layers.

At Diamond Creek, the Hurricane Fault has offset the formations in just such a way that the cliffs can be avoided entirely. For the entire twenty mile length of the road, there is nary a cliff to worry about, as the road follows the bottoms of desert arroyos and washes. The biggest worry is flash floods and mudflows, which can easily shut down the road in July and August during the monsoons (and badly inconvenience river rafters who plan to take out at Diamond Creek). Unfortunately, our storm had all the hallmarks of an intense monsoon storm, and for all we knew, the road would be closed, and the whole endeavor moot. We were running late anyway, and I couldn't know the status of the road until we reached the Peach Springs office of the Game and Fish Service of the Hualapai Nation. We got there, and the sign on the door told me the worst...

All my plans dashed. But what can you do but barge your way into the office and see if the road was really closed? And it turned out that their closure was cautionary, the sign left from earlier in the day when no one had checked the road conditions. The road was okay, and I was given the permit to take our group into the canyon (the form interestingly was a permit to trespass on Hualapai land). It was the only permit given that day, and we would have the Grand Canyon to ourselves! We started down the gravel road, the Colorado River twenty miles away.

One can certainly debate the idea that it's "cheating" to drive to the bottom of the canyon, and there is a certain validity that scenery that hasn't been "earned" by completing a stiff hike might end up being less appreciated. But many people can't handle the very strenuous hiking and many simply don't have the time. Driving down into the canyon is a unique opportunity to study the oldest rocks of the canyon, the ones that are the very hardest to access in any other situation.

Diamond Creek cuts through to the Granite Gorge Metamorphic Suite, which is the oldest group of rocks found in the American Southwest, dating to as early as 1.8 billion years ago. At roadside there are marvelous exposures of schist, gneiss, and pegmatite granite with bright shiny crystals of muscovite mica and quartz.
Outcrops of schist intruded by pink pegmatite granite
A bit farther up the canyon one can see the layers of the Tonto Group, a series of three formations called the Tapeats Sandstone, the Bright Angel Shale, and the Muav Limestone. The three layers formed as the Pacific Ocean transgressed and covered much of the western North American continent in Cambrian time, just over 500 million years ago. The Tapeats Sandstone sits atop the rocks of the Granite Gorge Metamorphic Suite.
The Great Unconformity: the vertically oriented rocks in the lower half of the photo are the ancient metamorphic schist and gneiss. Above the metamorphic rocks are horizontal layers of the Tonto Group.
That last sentence needs a bit more explanation. The boundary between the Tapeats and the Granite Gorge rocks is a profound unconformity representing a gap of more than a billion years. This unconformity is actually called the "Great Unconformity", and is an erosional surface that was witness to not one, but two major mountain-building events. The metamorphic rocks were once the core of a vast mountain range that formed 1.7 billion years ago when the North American continent collided with a set of two exotic terranes, the Yavapai block and the Mazatzal block. Imagine an island the size of California or New Zealand grinding into the edge of a continent along a subduction zone and you will get the picture. This massive mountain range eroded to a flat low-relief surface over the next few hundred million years.
The Great Unconformity up close. Dark brown conglomerate of the Tonto Group rests unconformably atop the ancient metamorphic rocks.
Later on, about a billion years ago, the continent stretched and broke apart, forming a series of fault-block mountain ranges that reached heights similar to the mountains in and around Death Valley National Park today. The rocks of these mountains are exposed in the easternmost part of the Grand Canyon, but they too were eventually eroded down to a low-relief surface as well, although small  ridges a few hundred feet high persisted. Along the lower reaches of Diamond Creek Road, you can lay your hand on a boundary between two rock sequences with a gap of more than a billion years between them. The unconformity can be seen in the photo above where a conglomerate rests on contorted metamorphic rocks in the bit of shadowed ledge. The "Diamond" of Diamond Creek is the uniquely shaped peak on the left side of the photograph. It is actually a sliver of rock caught between two branches of the Hurricane fault system.

Another layer that is prominently exposed along Diamond Creek Road is the Devonian-aged Temple Butte Limestone. Most park visitors never see it because in the eastern section of Grand Canyon National Park the Temple Butte is a discontinuous thin layer that is pretty well invisible from the rim. In western Grand Canyon it is more than 700 feet thick. It formed as a tidal estuary and tidal flats along the edge of the continent about 385 million years ago.
Temple Butte cliffs in Diamond Creek
We reached the end of the road where Diamond Creek flows into the Colorado River, the watercourse responsible for carving the Grand Canyon. It was a moment for the students to cavort in the river for a few minutes, and to wonder at the work the river has done. This spot is where many of those who've rafted the river through the Grand Canyon take out, a usually raucous process, but no one was about on this particular day.
The most "mysterious" aspect of Diamond Creek and Peach Springs Canyon is that the tributary canyons to this creek follow illogical pathways, and are actually older than the Grand Canyon itself! It's an odd problem. Along the upper reaches of Peach Springs Canyon, there are a series of Paleocene to Miocene-aged rocks clinging to the canyon walls. They once filled the canyon, meaning the canyon was carved prior to sixty million years ago, and the rivers that carved it flowed northeast, opposite of the Colorado River today. Apparently the land subsided so that the canyons were filled with sediment, and then they were exhumed when the modern Grand Canyon was carved, most likely within the last four or five million years. There has been more than one Grand Canyon carved through the Colorado Plateau!
Peach Springs Canyon  (red) and the odd channels (blue) that were carved millions of years prior to the Grand Canyon itself (they were buried and exhumed when the modern canyon was carved).
Near the bottom of the road we happened upon a Bighorn Sheep. They were once more common and figured prominently in the Ancestral Pueblo petroglyphs found throughout the region. 
Diamond Creek is a fascinating excursion. It is entirely on Hualapai lands, and they charge a fee of around $17 per person for permission to drive the road. I never mind paying the fee, because the tribe doesn't have many sources of income or all that many jobs on the reservation. The town of Peach Springs where the road starts has a very nice motel and restaurant (and not much else), and if you are a train lover, you'll be able to listen to them all night long. The rails are amongst the busiest you'll ever see. The town is on one of the last remaining stretches of the original Route 66.

Our crew was ready to get to camp, so we drove an hour or so to my brother's cabin where we would spend the next three nights.
Wait a minute. Showers, kitchen, hot tub? You call this roughing it?